The present invention relates to olefin polymerization processes using two or more different diimine nickel (II) or palladium (II) complexes for making polyolefins having multimodal molecular weight distributions. In another aspect, the present invention relates to novel catalyst systems comprising at least two different diimine nickel (II) or palladium (II) complexes. In yet another aspect, the present invention relates to unique polyethylenes and other polyolefins made using such catalyst systems.
It is well known that mono-1-olefins, such as ethylene and propylene, can be polymerized with catalyst systems employing transition metals such as titanium, vanadium, chromium, nickel and/or other metals, either unsupported or on a support such as alumina, silica, titania, and other refractory metals. Supported polymerization catalyst systems frequently are used with a co-catalyst, such as alkyl boron compounds and/or alkyl aluminum compounds and/or alkyl aluminoxy compounds. Organometallic catalyst systems, i.e., Ziegler-Natta-type catalyst systems, usually are unsupported and frequently are used with a co-catalyst, such as methylaluminoxane.
It is also well known that slurry polymerization processes, or loop polymerization processes, are relatively more commercially desirable than other polymerization processes. Furthermore, the type of polymerization process used can have an effect on the nature of the resultant polymer. For example, higher reactor temperatures can result in low catalyst system activity and productivity, as well as a lower molecular weight polymer product. Higher reactor pressures also can decrease the amount of desirable branching in the resultant polymer.
Most polymer products made by slurry processes, especially those polymer products made using supported chromium catalyst systems, have a broader molecular weight distribution (MWD) and, therefore, the polymer product is much easier to process into a final product. Polymers made by other processes, such as, for example, higher temperature and/or higher pressure solution processes, can produce polymers having a narrow molecular weight distribution; these polymers can be much more difficult to process into an article of manufacture.
Unfortunately, many homogeneous organometallic catalyst systems have a low activity, high consumption of very costly co-catalysts, like methylaluminoxane (MAO), and can produce low molecular weight polymers having a narrow molecular weight distribution. Furthermore, even though MAO can be necessary to produce a polymer with desired characteristics, an excess of MAO can result in decreased catalyst system activity. Additionally, these types of homogeneous catalyst systems preferably are used only in solution or gas phase polymerization processes.
In general, polyolefins produced by present-day processes comprise polymers having a distribution of different molecular weights. In many cases, a molecular weight distribution (MWD) is monomodal, meaning that the molecular weight distribution generally is characterized by having a single peak or mode, as reflected by the distribution of polymer molecular weights, for example, as shown by size exclusion chromatography (SEC) curves. Also, some polyolefins made according to present-day processes generally are characterized by having a narrow molecular weight distribution. While in some circumstances it may be desirable to have a polymer having a narrow molecular weight distribution or a monomodal molecular weight distribution, in other circumstances it may be advantageous to produce a polymer having a multimodal molecular weight distribution and/or a broad molecular weight distribution.
Polyolefins having a multimodal molecular weight distribution, such as polyethylene having a bimodal MWD, can be made into articles by a variety of methods, including but not limited to extrusion molding, thermoforming and rotational molding, and have advantages over typical polyolefins lacking a multimodal MWD. In particular, it has been observed that polyolefins having a multimodal MWD can be processed more easily. For example, they can be processed at a faster throughput rate with lower energy requirements, and at the same time such polymers tend to exhibit reduced melt flow perturbations. Polyolefins having a multimodal MWD can be preferred because of improved properties for applications such as blow molding and/or high strength firms. Polymers having a multimodal MWD generally are characterized by having more than one MWD peak, or in some cases by a broad MWD, as reflected by SEC curves.
Polyolefins having a multimodal MWD can be made by several different methods. In one method, a combination of two distinct and separate catalyst systems are used in the same polymerization reactor, wherein each catalyst system on its own is known to produce a polyolefin having a MWD that is different than the MWD of the polyolefin produced by the other catalyst system. When certain catalyst systems are used in combination, the resultant polyolefins can have a bimodal or multimodal molecular weight distribution. For example, titanium and chromium catalyst systems can be employed simultaneously in a single polymerization process in an attempt to produce a polyolefin having a bimodal molecular weight distribution. However, the use of two different catalyst systems in a single polymerization reactor does not necessarily guarantee the production of a polyolefin having a bimodal or multimodal MWD. Some polymerization reactions nonetheless can yield a polyolefin having a monomodal molecular weight distribution. Furthermore, in processes using two distinct catalyst systems, it usually is difficult to control catalyst system feed rates, and one catalyst system can inhibit or impair the activity of the other catalyst systems. Also, use of a combination of two distinct and separate catalyst systems in the same reactor can lead to the formation of polymer particles that are not uniform in size. Thus, segregation of the polymer product during storage and/or transfer can produce non-homogenous polymer products.
Another method for making polyolefin product having a bimodal MWD comprises combining or blending two separate polyolefins which have different molecular weight distributions. However, this requires at least one extra step, and likely requires extra equipment and/or manpower, in the process for making the final olefin product. Furthermore, problems discussed earlier regarding segregation also can occur in this method.
Another method for making multimodal MWD polyolefins in a single reactor polymerization process involves using a catalyst system comprising two or more catalytic sites, such as, for example, metallocenes wherein each site has different propagation and termination rate constants. Again, in certain circumstances, even catalyst systems that have two different catalytic sites can produce a unimodal molecular weight distribution. Also, such catalyst systems can have decreased catalytic activity.
Accordingly, a catalyst system is desired that produces polymers having bimodal or multimodal molecular weight distributions.
It is an object of this invention to provide novel catalyst systems useful for polymerization.
It is another object of this invention to provide catalyst systems which are relatively simple to make, have increased activity and increased productivity.
It is a further object of this invention to provide catalyst systems which have reduced co-catalyst consumption.
It is another object of this invention to provide polyethylene and other polymers that have a bimodal or multimodal molecular weight distribution. It is yet another object of this invention to provide a process for making polyolefins having a bimodal or multimodal molecular weight distribution. It is yet another object of this invention.
One aspect of this invention is a catalyst system comprising a first diimine nickel (II) or palladium (II) complex and a second diimine nickel (II) or palladium (II) complex, wherein the first diimine complex has a lower molecular weight and/or smaller ligand(s) than the second diimine complex. It is further contemplated that such a catalyst system can also comprise a suitable co-catalyst such as methylaluminoxane. Another aspect of this invention is a process for making polyethylenes and other polyolefins by contacting such a catalyst system with ethylene or other olefin monomer under polymerization conditions.
Another aspect of this invention is a novel polymer having a bimodal or multimodal molecular weight distribution and in which at least some polymer having a higher molecular weight is substantially branched, or at least not insubstantially branched, and at least some polymer having a lower molecular weight is substantially linear. Another aspect of this invention is a polymer having a bimodal or multimodal MWD in which branching is concentrated in the high molecular weight portion of the MWD. Another aspect of this invention is a process for making such polymers.
In accordance with this invention, catalyst systems comprising at least two different diimine nickel complexes or diimine palladium complexes are employed. Each diimine nickel or palladium complex can comprise additional ligands selected from the group consisting of xcex1-deprotonated-xcex2-diketones, xcex1-deprotonated-xcex2-ketoesters, halogens and mixtures thereof. Preferably, the diimine nickel complexes have a formula selected from the group consisting of Ni(NCRxe2x80x2C6R2H3)2 (E2C3Rxe2x80x32Q)2 and Ni(NCRxe2x80x2C6R2H3)2(E2C3Rxe2x80x32Q)X. Most preferably, the catalyst systems comprise two different diimine nickel (II) complexes, each of which has a formula selected from the aforesaid group. The catalyst systems further can comprise methylaluminoxane or other cocatalyst(s). Catalyst systems also can comprise other components, for example, the catalyst systems can include inorganic oxide supports. Exemplary inorganic supports include, but are not limited to, silica, alumina, titania, silica/alumina, silica/titania, silica/alumina/titania, aluminophosphate, and mixtures thereof. In particular, it can be advantageous to include aluminophosphate. Processes to make these catalyst systems also are provided.
Yet another aspect of this invention is a slurry polymerization process comprising contacting ethylene, and optionally one or more higher alpha-olefins, in a reaction zone with a catalyst system comprising at least two different diimine nickel complexes or diimine palladium complexes, which can further comprise additional ligands selected from the group consisting of xcex1-deprotonated-xcex2-diketones, xcex1-deprotonated-xcex2-ketoesters, halogens and mixtures thereof, in the presence of methylaluminoxane or other co-catalysts. In some embodiments of the invention, the catalyst systems can be heterogeneous.